482 lines
12 KiB
Go
482 lines
12 KiB
Go
package lz4block
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import (
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"encoding/binary"
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"math/bits"
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"sync"
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"github.com/pierrec/lz4/v4/internal/lz4errors"
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)
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const (
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// The following constants are used to setup the compression algorithm.
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minMatch = 4 // the minimum size of the match sequence size (4 bytes)
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winSizeLog = 16 // LZ4 64Kb window size limit
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winSize = 1 << winSizeLog
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winMask = winSize - 1 // 64Kb window of previous data for dependent blocks
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// hashLog determines the size of the hash table used to quickly find a previous match position.
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// Its value influences the compression speed and memory usage, the lower the faster,
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// but at the expense of the compression ratio.
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// 16 seems to be the best compromise for fast compression.
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hashLog = 16
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htSize = 1 << hashLog
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mfLimit = 10 + minMatch // The last match cannot start within the last 14 bytes.
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)
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func recoverBlock(e *error) {
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if r := recover(); r != nil && *e == nil {
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*e = lz4errors.ErrInvalidSourceShortBuffer
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}
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}
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// blockHash hashes the lower 6 bytes into a value < htSize.
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func blockHash(x uint64) uint32 {
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const prime6bytes = 227718039650203
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return uint32(((x << (64 - 48)) * prime6bytes) >> (64 - hashLog))
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}
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func CompressBlockBound(n int) int {
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return n + n/255 + 16
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}
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func UncompressBlock(src, dst, dict []byte) (int, error) {
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if len(src) == 0 {
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return 0, nil
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}
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if di := decodeBlock(dst, src, dict); di >= 0 {
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return di, nil
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}
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return 0, lz4errors.ErrInvalidSourceShortBuffer
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}
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type Compressor struct {
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// Offsets are at most 64kiB, so we can store only the lower 16 bits of
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// match positions: effectively, an offset from some 64kiB block boundary.
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//
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// When we retrieve such an offset, we interpret it as relative to the last
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// block boundary si &^ 0xffff, or the one before, (si &^ 0xffff) - 0x10000,
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// depending on which of these is inside the current window. If a table
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// entry was generated more than 64kiB back in the input, we find out by
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// inspecting the input stream.
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table [htSize]uint16
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// Bitmap indicating which positions in the table are in use.
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// This allows us to quickly reset the table for reuse,
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// without having to zero everything.
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inUse [htSize / 32]uint32
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}
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// Get returns the position of a presumptive match for the hash h.
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// The match may be a false positive due to a hash collision or an old entry.
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// If si < winSize, the return value may be negative.
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func (c *Compressor) get(h uint32, si int) int {
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h &= htSize - 1
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i := 0
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if c.inUse[h/32]&(1<<(h%32)) != 0 {
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i = int(c.table[h])
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}
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i += si &^ winMask
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if i >= si {
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// Try previous 64kiB block (negative when in first block).
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i -= winSize
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}
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return i
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}
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func (c *Compressor) put(h uint32, si int) {
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h &= htSize - 1
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c.table[h] = uint16(si)
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c.inUse[h/32] |= 1 << (h % 32)
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}
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func (c *Compressor) reset() { c.inUse = [htSize / 32]uint32{} }
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var compressorPool = sync.Pool{New: func() interface{} { return new(Compressor) }}
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func CompressBlock(src, dst []byte) (int, error) {
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c := compressorPool.Get().(*Compressor)
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n, err := c.CompressBlock(src, dst)
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compressorPool.Put(c)
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return n, err
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}
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func (c *Compressor) CompressBlock(src, dst []byte) (int, error) {
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// Zero out reused table to avoid non-deterministic output (issue #65).
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c.reset()
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// Return 0, nil only if the destination buffer size is < CompressBlockBound.
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isNotCompressible := len(dst) < CompressBlockBound(len(src))
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// adaptSkipLog sets how quickly the compressor begins skipping blocks when data is incompressible.
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// This significantly speeds up incompressible data and usually has very small impact on compression.
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// bytes to skip = 1 + (bytes since last match >> adaptSkipLog)
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const adaptSkipLog = 7
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// si: Current position of the search.
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// anchor: Position of the current literals.
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var si, di, anchor int
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sn := len(src) - mfLimit
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if sn <= 0 {
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goto lastLiterals
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}
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// Fast scan strategy: the hash table only stores the last 4 bytes sequences.
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for si < sn {
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// Hash the next 6 bytes (sequence)...
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match := binary.LittleEndian.Uint64(src[si:])
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h := blockHash(match)
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h2 := blockHash(match >> 8)
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// We check a match at s, s+1 and s+2 and pick the first one we get.
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// Checking 3 only requires us to load the source one.
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ref := c.get(h, si)
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ref2 := c.get(h2, si+1)
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c.put(h, si)
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c.put(h2, si+1)
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offset := si - ref
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if offset <= 0 || offset >= winSize || uint32(match) != binary.LittleEndian.Uint32(src[ref:]) {
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// No match. Start calculating another hash.
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// The processor can usually do this out-of-order.
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h = blockHash(match >> 16)
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ref3 := c.get(h, si+2)
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// Check the second match at si+1
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si += 1
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offset = si - ref2
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if offset <= 0 || offset >= winSize || uint32(match>>8) != binary.LittleEndian.Uint32(src[ref2:]) {
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// No match. Check the third match at si+2
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si += 1
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offset = si - ref3
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c.put(h, si)
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if offset <= 0 || offset >= winSize || uint32(match>>16) != binary.LittleEndian.Uint32(src[ref3:]) {
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// Skip one extra byte (at si+3) before we check 3 matches again.
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si += 2 + (si-anchor)>>adaptSkipLog
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continue
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}
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}
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}
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// Match found.
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lLen := si - anchor // Literal length.
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// We already matched 4 bytes.
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mLen := 4
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// Extend backwards if we can, reducing literals.
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tOff := si - offset - 1
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for lLen > 0 && tOff >= 0 && src[si-1] == src[tOff] {
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si--
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tOff--
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lLen--
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mLen++
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}
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// Add the match length, so we continue search at the end.
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// Use mLen to store the offset base.
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si, mLen = si+mLen, si+minMatch
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// Find the longest match by looking by batches of 8 bytes.
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for si+8 <= sn {
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x := binary.LittleEndian.Uint64(src[si:]) ^ binary.LittleEndian.Uint64(src[si-offset:])
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if x == 0 {
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si += 8
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} else {
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// Stop is first non-zero byte.
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si += bits.TrailingZeros64(x) >> 3
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break
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}
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}
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mLen = si - mLen
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if di >= len(dst) {
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return 0, lz4errors.ErrInvalidSourceShortBuffer
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}
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if mLen < 0xF {
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dst[di] = byte(mLen)
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} else {
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dst[di] = 0xF
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}
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// Encode literals length.
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if lLen < 0xF {
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dst[di] |= byte(lLen << 4)
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} else {
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dst[di] |= 0xF0
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di++
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l := lLen - 0xF
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for ; l >= 0xFF && di < len(dst); l -= 0xFF {
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dst[di] = 0xFF
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di++
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}
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if di >= len(dst) {
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return 0, lz4errors.ErrInvalidSourceShortBuffer
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}
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dst[di] = byte(l)
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}
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di++
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// Literals.
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if di+lLen > len(dst) {
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return 0, lz4errors.ErrInvalidSourceShortBuffer
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}
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copy(dst[di:di+lLen], src[anchor:anchor+lLen])
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di += lLen + 2
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anchor = si
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// Encode offset.
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if di > len(dst) {
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return 0, lz4errors.ErrInvalidSourceShortBuffer
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}
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dst[di-2], dst[di-1] = byte(offset), byte(offset>>8)
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// Encode match length part 2.
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if mLen >= 0xF {
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for mLen -= 0xF; mLen >= 0xFF && di < len(dst); mLen -= 0xFF {
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dst[di] = 0xFF
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di++
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}
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if di >= len(dst) {
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return 0, lz4errors.ErrInvalidSourceShortBuffer
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}
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dst[di] = byte(mLen)
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di++
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}
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// Check if we can load next values.
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if si >= sn {
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break
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}
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// Hash match end-2
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h = blockHash(binary.LittleEndian.Uint64(src[si-2:]))
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c.put(h, si-2)
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}
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lastLiterals:
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if isNotCompressible && anchor == 0 {
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// Incompressible.
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return 0, nil
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}
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// Last literals.
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if di >= len(dst) {
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return 0, lz4errors.ErrInvalidSourceShortBuffer
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}
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lLen := len(src) - anchor
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if lLen < 0xF {
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dst[di] = byte(lLen << 4)
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} else {
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dst[di] = 0xF0
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di++
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for lLen -= 0xF; lLen >= 0xFF && di < len(dst); lLen -= 0xFF {
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dst[di] = 0xFF
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di++
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}
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if di >= len(dst) {
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return 0, lz4errors.ErrInvalidSourceShortBuffer
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}
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dst[di] = byte(lLen)
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}
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di++
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// Write the last literals.
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if isNotCompressible && di >= anchor {
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// Incompressible.
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return 0, nil
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}
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if di+len(src)-anchor > len(dst) {
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return 0, lz4errors.ErrInvalidSourceShortBuffer
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}
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di += copy(dst[di:di+len(src)-anchor], src[anchor:])
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return di, nil
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}
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// blockHash hashes 4 bytes into a value < winSize.
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func blockHashHC(x uint32) uint32 {
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const hasher uint32 = 2654435761 // Knuth multiplicative hash.
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return x * hasher >> (32 - winSizeLog)
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}
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type CompressorHC struct {
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// hashTable: stores the last position found for a given hash
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// chainTable: stores previous positions for a given hash
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hashTable, chainTable [htSize]int
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needsReset bool
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}
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var compressorHCPool = sync.Pool{New: func() interface{} { return new(CompressorHC) }}
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func CompressBlockHC(src, dst []byte, depth CompressionLevel) (int, error) {
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c := compressorHCPool.Get().(*CompressorHC)
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n, err := c.CompressBlock(src, dst, depth)
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compressorHCPool.Put(c)
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return n, err
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}
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func (c *CompressorHC) CompressBlock(src, dst []byte, depth CompressionLevel) (_ int, err error) {
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if c.needsReset {
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// Zero out reused table to avoid non-deterministic output (issue #65).
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c.hashTable = [htSize]int{}
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c.chainTable = [htSize]int{}
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}
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c.needsReset = true // Only false on first call.
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defer recoverBlock(&err)
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// Return 0, nil only if the destination buffer size is < CompressBlockBound.
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isNotCompressible := len(dst) < CompressBlockBound(len(src))
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// adaptSkipLog sets how quickly the compressor begins skipping blocks when data is incompressible.
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// This significantly speeds up incompressible data and usually has very small impact on compression.
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// bytes to skip = 1 + (bytes since last match >> adaptSkipLog)
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const adaptSkipLog = 7
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var si, di, anchor int
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sn := len(src) - mfLimit
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if sn <= 0 {
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goto lastLiterals
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}
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if depth == 0 {
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depth = winSize
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}
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for si < sn {
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// Hash the next 4 bytes (sequence).
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match := binary.LittleEndian.Uint32(src[si:])
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h := blockHashHC(match)
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// Follow the chain until out of window and give the longest match.
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mLen := 0
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offset := 0
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for next, try := c.hashTable[h], depth; try > 0 && next > 0 && si-next < winSize; next, try = c.chainTable[next&winMask], try-1 {
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// The first (mLen==0) or next byte (mLen>=minMatch) at current match length
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// must match to improve on the match length.
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if src[next+mLen] != src[si+mLen] {
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continue
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}
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ml := 0
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// Compare the current position with a previous with the same hash.
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for ml < sn-si {
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x := binary.LittleEndian.Uint64(src[next+ml:]) ^ binary.LittleEndian.Uint64(src[si+ml:])
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if x == 0 {
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ml += 8
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} else {
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// Stop is first non-zero byte.
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ml += bits.TrailingZeros64(x) >> 3
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break
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}
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}
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if ml < minMatch || ml <= mLen {
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// Match too small (<minMath) or smaller than the current match.
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continue
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}
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// Found a longer match, keep its position and length.
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mLen = ml
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offset = si - next
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// Try another previous position with the same hash.
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}
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c.chainTable[si&winMask] = c.hashTable[h]
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c.hashTable[h] = si
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// No match found.
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if mLen == 0 {
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si += 1 + (si-anchor)>>adaptSkipLog
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continue
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}
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// Match found.
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// Update hash/chain tables with overlapping bytes:
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// si already hashed, add everything from si+1 up to the match length.
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winStart := si + 1
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if ws := si + mLen - winSize; ws > winStart {
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winStart = ws
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}
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for si, ml := winStart, si+mLen; si < ml; {
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match >>= 8
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match |= uint32(src[si+3]) << 24
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h := blockHashHC(match)
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c.chainTable[si&winMask] = c.hashTable[h]
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c.hashTable[h] = si
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si++
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}
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lLen := si - anchor
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si += mLen
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mLen -= minMatch // Match length does not include minMatch.
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if mLen < 0xF {
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dst[di] = byte(mLen)
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} else {
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dst[di] = 0xF
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}
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// Encode literals length.
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if lLen < 0xF {
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dst[di] |= byte(lLen << 4)
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} else {
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dst[di] |= 0xF0
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di++
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l := lLen - 0xF
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for ; l >= 0xFF; l -= 0xFF {
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dst[di] = 0xFF
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di++
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}
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dst[di] = byte(l)
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}
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di++
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// Literals.
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copy(dst[di:di+lLen], src[anchor:anchor+lLen])
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di += lLen
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anchor = si
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// Encode offset.
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di += 2
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dst[di-2], dst[di-1] = byte(offset), byte(offset>>8)
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// Encode match length part 2.
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if mLen >= 0xF {
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for mLen -= 0xF; mLen >= 0xFF; mLen -= 0xFF {
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dst[di] = 0xFF
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di++
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}
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dst[di] = byte(mLen)
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di++
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}
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}
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if isNotCompressible && anchor == 0 {
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// Incompressible.
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return 0, nil
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}
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// Last literals.
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lastLiterals:
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lLen := len(src) - anchor
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if lLen < 0xF {
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dst[di] = byte(lLen << 4)
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} else {
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dst[di] = 0xF0
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di++
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lLen -= 0xF
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for ; lLen >= 0xFF; lLen -= 0xFF {
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dst[di] = 0xFF
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di++
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}
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dst[di] = byte(lLen)
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}
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di++
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// Write the last literals.
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if isNotCompressible && di >= anchor {
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// Incompressible.
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return 0, nil
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}
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di += copy(dst[di:di+len(src)-anchor], src[anchor:])
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return di, nil
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}
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